1. Central Animal Laboratory, Catholic University of Nijmegen, PO Box 9101, 6500 HB Nijmegen, The Netherlands; Netherlands Institute for Dairy Research (NIZO), Department of Microbiology, PO Box 20, 6710 BA Ede, The Netherlands
2. Central Animal Laboratory, Catholic University of Nijmegen, PO Box 9101, 6500 HB Nijmegen, The Netherlands
3. Department of Human Nutrition, Wageningen Agricultural University, PO Box 8129, 6700 EV Wageningen, The Netherlands; Department of Laboratory Animal Science, State University of Utrecht, PO Box 80166, 3508 TD Utrecht, The Netherlands